In order to assess inventive step in an objective and predictable manner, the so-called "problem-and-solution approach" should be is applied. Thus deviation from this approach should be exceptional.
In the problem-and-solution approach, there are three main stages:
determining the "closest prior art",
establishing the "objective technical problem" to be solved, and
considering whether or not the claimed invention, starting from the closest prior art and the objective technical problem, would have been obvious to the skilled person.